7 Multiservice switching forum: MULTI-PLANE system architecture
7.3 Interfaces and reference points
A Multiservice network requires the interaction of multiple switching systems operating at several planes in order to deliver a complete set of user services. The role of the MSF is to specify the protocols and interfaces used within an MSS (Multiservice Switching System). The inter-plane interfaces between the Management, Control, Switching and Adaptation Planes are described in clause 7.3.1. While interfaces between the Application and Control Planes are beyond the scope of the first release of the MSF Architecture, future releases of the architecture are expected to look at APIs and protocols between the Application and Control Planes.
The MSF also has a role to aid in the selection of protocols and interfaces to be used between MSSs. These protocols and interfaces, defined outside the MSF by other standardization organizations, are described in clause 7.3.2. In particular, Control Plane to Control Plane protocols are defined by the ATM Forum (UNI, PNNI, AINI), ITU-T (DSS1, DSS2, ISUP, B-ISUP, BICC, H.323, Q.2111) and the IETF (SIP, MPLS, OSPF, BGP, SIGTRAN). Adaptation Plane to Adaptation Plane protocols are also defined by various bodies.
This clause describes reference points between functions across a boundary between planes in the MSF Architecture for a potential requirement to specify a protocol and an interface to implement the interaction between the respective functional blocks. The specifications for these inter-plane interfaces and protocols will be defined in the respective MSF Implementation Agreement for those interfaces. See clause 5.6 for additional information.
7.3.1 Inter-plane interactions
This clause describes the reference points from figure 6.2 that cross planar boundaries. These are sometimes called vertical interfaces.
7.3.1.1
Adaptation Plane - Control Plane
The Adaptation Plane shall support a range of service interfaces between Control Planes, including UNIs and NNIs for Voice, IP, Frame Relay, Circuit Emulation and ATM services. Each service has a distinct set of attributes and signalling requirements that must be supported by the Adaptation-Control Plane interface (np or sp reference points). The MSF shall define a set of Adaptation-Control Plane protocols that allow the support of this full range of services. These protocols shall enable both the relaying of service signalling information as well as control of the Adaptation Plane functions. In the first release, MEGACO/H.248 is the protocol for providing Voice and N-ISDN services between the Adaptation and Control Planes, while GSMPv3 is the protocol for providing frame relay, ATM, MPLS and circuit emulation services.
Edge Control Signalling reference point (np):
This signalling association provides the signalling necessary to request the creation, modification, status or termination of a designated logical port session instance. One or more media could be handled within a logical port session. These media could be multiplexed and carried on a single path through the switch function. The signalling messages have the capability of adding and deleting media streams to the logical port session instance.
Switch Port Signalling reference point (sp):
Via information flowing across this reference point the virtual switch function conveys information to the logical port function that establishes a correlation between labels associated with external data-flows. An example is the conveyance of traffic descriptor information to enable the LPF (Logical Port Function) to perform policing, shaping and provide the required quality of service. Furthermore, some traditional broadband services may also have the adaptation control realized across the sp interface. Examples of these services would be ATM, FR, CES, and MPLS.
The protocol requirements for interfaces supporting the np and sp reference points are specified in clause 3.4 of [49].
7.3.1.2
Switching Plane - Control Plane
The MSF architecture supports multiple independent controllers controlling logical switch partitions of a physical switch. The VSF (Virtual Switch Function) is responsible for insuring the integrity of a switch that has been partitioned into multiple virtual switches.
The protocol requirements are specified in clause 3.4 of [49].
Virtual Switch Control Signalling reference point (vsc):
This signalling association provides for the creation, modification, status, or termination of a connection path between logical ports. A connection may be point-to-point, point-multipoint, multipoint-point, or broadcast. Point-point connections may be unidirectional or bi-directional.
7.3.1.3
Switching Plane - Adaptation Plane
The Adaptation and Switching Planes are coordinated by the Control Plane via collaboration of the various control functions (NSICF, NECF, BCF, VSCF) as depicted by the MSF functional architecture. No reference point is defined between the Switching and Adaptation Planes in this version of the MSF Architecture.
7.3.1.4
Control Plane - Application Plane
The MSF architecture supports a multiservice switching system communicating with applications over the sa and sg reference points. These reference points also allow applications in the Applications Plane to access the Control Plane functionality. Communication between the Control and Applications Plane may use open software APIs or open standardized physical interfaces. The MSF has not chosen to address the standards associated with these reference points as part of Release 1, but they are candidates for being addressed in future releases. It is assumed that where enhanced "off-board" services are required for Release 1, existing open standards will be utilized (e.g. AIN, CS-1, etc.).
Service Access Signalling reference point (sa):
This signalling association provides the transport of service events, requests and responses between the NSICF (Network Service Instance Control Function) and the SFGF (Service Feature Gateway Function).
Service Access Gateway Signalling reference point (sg):
This signalling association provides the transport of service requests and responses between external customers or networks and the applications plane via the SGF and the SFGF.
Example of sg applied to SS7 INAP/ SIGTRAN mapping:
IETF SIGTRAN SCTP, being developed by the IETF to transport SS7 INAP signalling messages over an IP transport network, is an example of a "sg" interface. SS7 INAP signalling arriving at SGF may be transported using IETF SIGTRAN SCTP to a SFGF.
7.3.1.5
Management Plane- Switching and Adaptation Plane
The MSF leverages several industry standards and agreements regarding management. What is specific for the MSF is the standardization of new MIBs required by the MSF Architecture and new MSF specific management interfaces (over reference points sm and vsm as shown in figure 6.3). The only interface between the management and the Adaptation and Switching Planes is provided through MIBs that offer an object abstraction of the Adaptation Plane through proprietary agents. The MSF will use industry standard network management protocols to implement the sm and vsm
reference points and will specify protocol independent MIBs for the VS MIB and Partition Management (Partitioning MIB).
Three reference points in the Management Plane are identified. In addition, management flows occur at the SCI interface as well:
Switch Management reference point (sm):
The sm reference pointenables the management of the switch through the following MIBs. These MIBs are referred to as the Switch Management MIB:
• The Switch Partitioning MIB is used specifically for configuration of VSs. Configuring a partition entails the specification of the resources that it contains.
• The GSMPv3 MIB is used specifically for management of the SCI instances presented by a switch to a controller.
Other standardized MIBs such as MIB I, MIB II, AToM MIB are used for the exchange of aggregated FCAPS management information relating to the underlying switching function (physical switch).
An interface for the sm reference point is subject to MSF standardization.
Virtual Switch Management reference point (vsm):
The vsm reference point is presented once for each VS (Virtual Switch) created using the Partitioning MIB. Each instance of the vsm (each VS MIB) presents, to a sub-ordinate management function, a subset of the Switch MIB, which contains FCAPS information for one VS.
An interface for the vsm reference point is subject to MSF standardization.
7.3.1.6
Management - Control Plane
The MSF Architecture is intended to allow for widest possible choice of technologies to be used in the Control Plane. The development of new controllers and their standardization is though beyond the scope of MSF in Release 1.
Virtual Switch Control Management reference point (vscm):
Each sub-ordinate management function manages its corresponding VSCF (Virtual Switch Control Function) over the vscm. The management of controllers is specific to the type of controller.
An interface for the vscm reference point is NOT subject to MSF standardization in Release 1.
7.3.2 Intra-plane interactions
This clause describes the reference points from figure 6.2 within a single plane. These are sometimes called horizontal interfaces.
7.3.2.1
Control Plane
7.3.2.1.1
Control - Control Plane, between same functions
This clause describes reference points within the Control Plane categorized by whether they exist between the same function or between different functions.
At least the following Control-Control Plane interfaces and protocols operate between switching systems, as defined by the following bodies:
• ATM Forum (e.g. UNI, PNNI, AINI)
• IETF (e.g. MPLS LDP, OSPF, BGP, RSVP, IS-IS, RIP) • ITU-T (e.g. DSS1, DSS2, ISUP, B-ISUP. BIC, TUP)
The MSF shall work to ensure that the routing and connection control protocols specified by these bodies can be supported by the MSF architecture. Similarly, the MSF shall work to support a relevant set of UNIs, SNIs and NNIs defined by these bodies.
Inter-NSICF Signalling reference point (ia):
This signalling association provides the necessary signalling to request the establishment, modification and release of service instances and to transfer service instance information between instances of the NSICF (Network Service Instance Control Function).
Inter-MSS Bearer Control Signalling reference point (ic):
This signalling association provides the necessary signalling to request the establishment, modification and release of consistent local bearer mappings (e.g. MPLS LDP) between instantiations of the BCF (Bearer Control Function).
External control signalling (ix):
This signalling association provides the necessary signalling for interconnecting different networks and/or technologies. It is anticipated that this signalling will support both call control and bearer control information. Some examples of interfaces supporting the "ix" information are SS7-ISUP signalling links, ISDN D-channel, VPI/VCI 0/5 for ATM SVCs, DLCI 0 for FR SVCsand BGP.
All of these reference points address the information carried in layer 3 signalling and routing protocols assigned with them in a physical realization. The ix reference point interfaces to the SGF (Signalling Gateway Function), which then interfaces to the NSICF (Network Service Instance Control Function) via a SGF using the st reference point. The SGF handles the lower layers of the signalling and routing protocols. Often, the MSF architectural model inserts an SGF at the interface between a network and a subscriber or between provider networks in order to establish the correct reference point model.
7.3.2.1.2
Control Plane, between different functions
Bearer Control Signalling reference point (bc):
This signalling association provides the necessary signalling interaction between the NSICF (Network Service Instance Control Function) and the BCF (Bearer Control Function) to request and indicate the establishment, modification or release of an end to end bearer between MSS edge nodes.
Signalling Transport Signalling reference point (st):
This signalling association provides the transport of incoming and outgoing call and bearer signalling between the Signalling Gateway Function and Network Service Instance Control Function.
Example of st applied to PSTN-ATM Interworking:
SIGTRAN SCTP, being developed by the IETF, is one example of a "st" interface. SS7 and ISDN D-channel signalling are examples of common channel signalling that could be transported using IETF SIGTRAN SCTP or Q.2111.
A second example of st applied to PSTN-ATM Interworking:
Channel Associated Signalling (CAS) could be an example of signalling obtained from a Signalling Gateway Function (that is realized as part of the physical port) that could be transported over st.
Example of st applied to ATM SVC Service:
The SGF relays customer ATM SVC signalling from the assigned VPI/VCI to the NSICF that handles ATM SVC services. The customer initiates a request for a SVC via UNI signalling to an edge ATM switch. A cross-connect in the switch redirects the UNI to the SVC service controller where proxy signalling will be used to fulfil the SVC request. This cross-connect is a physical realization of the SGF where the SGF uses its relay capability. The redirection of the UNI constitutes another example of the signalling transport (st) interface between the SGF and the SVC service controller. See clause 4.2.1 for more detail.
Example of st applied to Frame Relay trunking over ATM:
In a similar fashion to the ATM SVC service interworking example, a logical instantiation of st would be the transport of Frame Relay DLCI 0 signalling (which is logically separate from the bearer) from the SGF on a physical port to the control function that governs the service on the logical port function of that physical port. .See clause 4.2.6 of [49] for more detail.
Network Service Instance Control Function (NSICF) to Virtual Switch Control Function (VSCF) reference point (bs"):
This reference point provides a means for the NSICF to control the VSF (via the VSCF) for the purpose of making a nodal level cross connect. It is used only when a BCF is not used for setting up an end to end bearer. Certain implementations of the MSF architecture may include both a bs and a bs' reference point simultaneously. However, each reference point would be uniquely responsible for its own separate set of end to end connections.
Bearer Control Function (BCF) to Virtual Switch Control Function reference point (VSCF) (bs):
This reference point provides a means for the BCF to control the VSF (via the VSCF) for the purpose of making a nodal level cross connect.
Network Service Instance Control Function (NSICF) to Network Edge Control Function reference point (mb):
A candidate reference point to be defined in a future MSF release.
7.3.2.2
Switching - Switching Plane (between same functions )
MSF has so far not considered an interface between like-to-like Switching - Switching Planes, since it is assumed that communication between Switch Functions interact over the Adaptation Plane.
7.3.2.3
Adaptation - Adaptation Plane (between same functions)
The Adaptation-Adaptation Plane interfaces specify the link-by-link bearer protocol stacks used by the various services. The intra MSS interfaces standardized by the MSF shall not hinder the full set of bearer capabilities that a Multiservice Switching System is expected to support.